1. Field of the Invention
The present invention relates to a clamping device in a main shaft driving device for a machine tool. More particularly, the present invention relates to a clamping device in a machine-tool main shaft driving device that rotationally drives by a driving device a main shaft rotatably supported by a frame and having a member that is rotationally driven secured to one end thereof. This clamping device includes a braking member, a pressure chamber, and an operating fluid supplying mechanism. The braking member is provided at a side of the frame, and has a pressure-receiving surface and a braking portion. The pressure-receiving surface receives pressure of an operating fluid. The braking portion causes a press-contact force to act upon the main shaft. The pressure chamber is formed by the pressure-receiving surface and the frame. The operating fluid supplying mechanism is provided for supplying the operating fluid from an operating fluid supply source to the pressure chamber. In this clamping device, the press-contact force is caused to act upon the main shaft by displacing the braking portion towards the main shaft by the pressure of the operating fluid that is supplied to the pressure chamber.
2. Description of the Related Art
As a main shaft driving device used in a machine tool, there is a rotation indexing table device that rotationally drives a circular table, on which, for example, a workpiece is placed, through a main shaft. The rotation indexing table device is used for indexing an angular position of the main shaft (circular table) by rotationally driving the main shaft, and processing a workpiece at the indexed angular position. Therefore, the rotation indexing table device includes a clamping device for maintaining the indexed angular position.
A related art of such a rotation indexing table device including a clamping device described above is discussed in Japanese Unexamined Patent Application Publication No. 2006-95668 (Patent Document 1). As driving means of a circular table, the rotation indexing table device in Patent Document 1 uses a direct-drive motor (hereunder referred to as “DD motor”) that rotationally drives the main shaft without using drive transmitting means such as a gear. In addition, the rotation indexing table device in Patent Document 1 includes as a clamping device a disc clamping device in which a brake plate (clamp disc) is mounted to the circular plate (secured to the main shaft that is rotationally driven by the DD motor) so as to be incapable of relative rotation; and in which a piston is urged towards the brake plate by operating fluid and the brake plate is held by cooperation of a frame and the piston.
More specifically, the clamping device in the rotation indexing table device in Patent Document 1 switches between a clamped state and an unclamped state of the circular table by introducing operating fluid. The piston is normally urged towards the brake plate by a compression spring, and stops rotation of the table by friction as a result of contacting the brake plate while introduction of operating fluid is stopped. When the table is rotated, the operating fluid is introduced, and the piston is moved against urging force of the compression spring to set the circular table in the unclamped state.
Accordingly, in the related main shaft driving device, the clamping device is generally set in the unclamp state when the main shaft is rotated. Even when a finishing operation is performed while continuously rotating the workpiece, the operation is performed while the clamping device is in the unclamp state.
When the finishing operation is performed while continuously rotating the workpiece, the main shaft driving device receives a load from a cutter that processes the workpiece. This load constantly varies even under a constant processing condition. Therefore, rotation state of the main shaft driving device is affected by the variations in the load that is received from the cutter.
When the DD motor is used as the driving means of the main shaft as in the rotation indexing table device discussed in Patent Document 1, control of the DD motor is carried out while correcting deviation of rotational angle (amount of rotation) of the main shaft driving device occurring due to the main shaft driving device being influenced by the load that it receives from the cutter as mentioned above, on the basis of feedback of the rotational angle (the amount of rotation) from a rotation detector of the main shaft driving device. However, due to the variations in the load received from the cutter, the rotation of the DD motor resulting from the control may not be able to follow the deviation. Therefore, the rotation of the DD motor is not necessarily uniform, and may include pulsation. In addition, when a speed reducer including gears is used as the driving means of the main shaft, the rotation state of the main shaft may include pulsation in a backlash range of the gears due to the influence of the variations in load that is received from the cutter.
As a result, pulsation occurs in the rotation state of the main shaft driving device, thereby deteriorating surface roughness of a finished surface of the workpiece.
When the clamping device is in the unclamp state, the rigidity of the main shaft of the main shaft driving device primarily depends upon the rigidities of the bearings. The rigidities of the bearings depend largely upon compression of the bearings. Therefore, when the rigidity of the main shaft is required, the compression of the bearings is made large. However, in order to withstand the large compression, the bearings themselves need to be large. When the bearings are made large, deflection precision is reduced. Therefore, indexing precision is reduced, as a result of which processing precision is reduced. In addition, since the larger the diameters of the bearings, the higher the cost, manufacturing cost of the device is increased.